1. Field of the Invention
The present invention relates to a plasma apparatus used as a CVD (Chemical Vapor Deposition) apparatus, an etching apparatus and the like.
2. Description of the Prior Art
Since a plasma apparatus utilizing an electron cyclotron resonance can form a plasma having a high activity at a low gas pressure and bring out a plasma flow having a large diameter, it can be applied to the formation of a thin film, etching and the like in a highly integrated semi-conductor element and the like. And, its investigation and development are progressing.
FIG. 1 is a longitudinal sectional view showing a conventional plasma apparatus constructed as a thin film forming apparatus. Reference numeral 31 designates a plasma chamber. Said plasma chamber 31 is provided with a microwave inlet 31c sealed up with a silica glass plate 31b at a center of an upper wall thereof and a plasma outlet window 31d at a position opposite to said microwave inlet 31c at a center of a lower wall thereof, said microwave inlet 31c being connected with one end of a wave guide 32, which is connected with a microwave generator (not shown) at the other end, a sample chamber 33 being arranged so as to face to the plasma outlet window 31d, and an exciting coil 34 being arranged concentrically with said sample chamber 33 so as to surround from the plasma chamber 31 to one end portion of the wave guide 32 connected with the plasma chamber 31.
The sample chamber 33 is provided with a sample stage 35 arranged at a position opposite to the plasma outlet window 31d, a sample 100, such as a disc wafer, being placed on the sample stage 35 as it is or detachably by means of a static absorption and the like, and the sample chamber 33 being provided with an exhaust port 33a, which is connected with an exhauster (not shown), opened at a lower wall thereof. In addition, reference numerals 31g, 33g designate material gas supply pipes, reference numeral 31e designating a water-supply system of a cooling water, and reference numeral 31f designating a drain system of the cooling water.
With such a thin film forming apparatus, the plasma chamber 31 and the sample chamber 33, which have been set at a desired vacuosity, are supplied with a material gas, a high-frequency electric field due to a microwave being applied in the plasma chamber 31 while forming a magnetic field by the exciting coil 34, to form a plasma, the resulting plasma being led out to the vicinity of the sample 100 on the sample stage 35 within the sample chamber 33 from the plasma chamber 31 through the plasma outlet window 31d by a divergent magnetic field formed by the exciting coil 34, and a surface reaction due to ions and radical particles in the plasma flow being brought on a surface of the sample 100 to form a film on the surface of the sample 100 (Japanese Patent Application Laid-Open No. 155535/1981).
However, with such the conventional plasma apparatus, the plasma brought about in the plasma chamber 31 is extracted to a side of the sample 100 within the sample chamber 33 through the plasma outlet window 31d along a line of magnetic force of the divergent magnetic field formed by the exciting coil 34 but the plasma generated in the plasma chamber 31 is not uniform, that is, a plasma density at a central portion of the plasma chamber 31 becomes higher than that at a periphery of the plasma chamber 31. This uneven distribution of plasma density has an enlarged influence upon the sample 100 by the divergent magnetic field as it is. In the case where the sample 100 is arranged in a concentrical manner with the plasma outlet window 31d, a difference occurs between the central portion and the periphery of the sample 100 in deposition rate, whereby making a film thickness uneven.
It is a reason why the uneven distribution of the plasma generated in the plasma chamber 31 is projected toward the sample 100 by the divergent magnetic field as it is that electrons in the plasma take a cyclotron movement (Larmor movement) having a cyclotron radius of several tens microns to several hundreds microns around the divergent magnetic field while ions in the plasma follow the electrons by an electric field having an amphoteric polarity generated between them and the electrons, so that the plasma is restricted by the line of magnetic force as a whole.
FIG. 2 is a diagram showing a positional relation between the divergent magnetic field formed by the exciting coil 34 and the sample 100. As obvious from FIG. 2, the line of magnetic force of the divergent magnetic field formed by the exciting coil 34 is expanded as it approaches to the sample 100 through the plasma outlet window 31d, so that the uneven distribution of the plasma generated in the plasma chamber 31 is projected on the sample 100 in an enlarged manner. Accordingly, in the case where a film is deposited, the uneven film thickness can not be avoided.
Such an uneven film thickness or etching depth has been found not only in the thin film forming apparatus but also in the case where it is used as an etching apparatus (Japanese Patent Application Laid-Open No. 47728/1984).